Apparatus and method for contactless transfer and soldering of chips using a flash lamp

ABSTRACT

A method and apparatus for soldering a chip ( 1   a ) to a substrate ( 3 ). A chip carrier ( 8 ) is provided between a flash lamp ( 5 ) and the substrate ( 3 ). The chip ( 1   a ) is attached to the chip carrier ( 8 ) on a side of the chip carrier ( 8 ) facing the substrate ( 3 ). A solder material ( 2 ) is disposed between the chip ( 1   a ) and the substrate ( 3 ). The flash lamp ( 5 ) generates a light pulse ( 6 ) for heating the chip ( 1   a ). The heating of the chip ( 1   a ) causes the chip ( 1   a ) to be released from the chip carrier ( 8 ) towards the substrate ( 3 ). The solder material ( 2 ) is at least partially melted by contact with the heated chip ( 1   a ) for attaching the chip ( 1   a ) to the substrate ( 3 ).

TECHNICAL FIELD AND BACKGROUND

The present disclosure relates to soldering, in particular to an apparatus and method for soldering chips to a substrate.

Simple flexible systems, such as logic functions with transistors or optoelectronic devices, can in principle be fully printed on a substrate (e.g. foil or rigid board). However, for more complex systems, there is a need to develop hybrid systems in which printed circuitry is combined with silicon-based integrated circuits or surface mounted device (SMD) components, referred herein as chip components or simply “chips”. To functionalize the device, multiple chip components, often having different dimensions, may need to be interconnected to the circuit tracks on the substrate, e.g. printed or etched copper circuits. This can be realized for example using oven reflow soldering, conducting adhesive bonding or face-up chip integration. However, these processes are considered time consuming and/or incompatible with low-cost polyester foils having low decomposition temperatures. Especially for soldering process, commonly used polymer substrates tend to degrade and deform under thermal load above 150° C.

For example, reflow soldering can typically be used for interconnecting thick chips on a rigid substrate board such as FR4 or ceramics. However, reflow soldering, is poorly compatible with low-cost flexible foils and roll-to-roll (R2R) processing because it requires maintaining the whole board above the liquidus temperature of the solder which is generally above 200° C. for long holding time. This typically results in a time-consuming process using large in-line ovens, often having multiple loops. The long holding time may also cause deformation or degradation of the flexible foil itself or degradation of its organic surface coatings or adhesives. It is considered impossible to oven reflow conventional solder on low-cost polyester foils, such as polyethylene terephthalate (PET), by using industry standard lead-free alloys because PET has a maximum processing temperature of around 120° C.-150° C., which is much lower than the liquidus temperature of these solders (>200° C.).

As an alternative, for example, infrared (IR) heating can be used with comparable soldering time. For example, infrared laser spots can be used to heat each solder connection sequentially. However, in laser spot soldering, the small spot area may require precise positioning of the spot for each component. Furthermore, applying this technology in a R2R process is challenging as the laser spot needs to align to multiple chips on a moving substrate. Furthermore, the process can be time consuming. Accordingly, some of these methods may still be based on stop and go.

As another alternative, large area illumination by a high-energy light pulse of a flash lamp can be used. For example, an article in Electronic Materials Letters, Vol. 10, No. 6 (2014), pp. 1175-1183 by Van den Ende et al. describes Large area photonic flash soldering of thin chips on flex foils for flexible electronic systems. Advantageously, when the timescale of the heating pulse is short enough to avoid diffusive heating of the flexible polymer substrate, components can be soldered at temperatures higher than the maximum processing temperature of the foils. However, if the absorption of light by the (foil) substrate and/or components differs, this can lead to selective heating. Furthermore, electronic devices, generally consist of multiple chip components. This may lead to further differences in heating behaviour for the different components which makes the temperature and solder process difficult to control.

Accordingly, there remains a need for improvement in the soldering of chips to a substrate, e.g. faster, more reliable, compatible with flexible foil substrates, roll-to-roll processing, and different chips and/or substrates.

SUMMARY

One aspect of the present disclosure may be embodied as a method for soldering a chip to a substrate. The method comprises providing a chip carrier between a flash lamp and the substrate. The chip is attached to the chip carrier on a side of the chip carrier facing the substrate. A solder material is disposed between the chip and the substrate. For example the solder material can be provided on the substrate where the chips are to be placed, at an underside of the chips, or both. A light pulse is generated with the flash lamp for heating the chip. The heating of the chip causes the chip to be released from the chip carrier towards the substrate. Furthermore, the solder material is at least partially melted by contact with the heated chip for attaching the chip to the substrate after re-solidification of the solder material.

Advantageously, the light pulse can be used for both releasing the chips from the carrier substrate and heating the chips for soldering. For example, heating of the chips may cause the chips to be released from the substrate by partial decomposition of the carrier and/or a sacrificial adhesive layer between the chip and carrier. Furthermore, decomposition by heating may cause rapid gas formation that can increase impulse of the chip thereby launching it towards the substrate. The light pulse from a flash lamp is found particularly advantageous in this respect because it can deliver a high energy light pulse with over a relatively large area with homogenous intensity and with a relatively long pulse length e.g. on the order of milliseconds. Additionally, a masking device may be used to pattern the light from the flash lamp, e.g. to irradiate only the positions of the chips and leave the rest of the carrier and substrate unaffected.

Flash lamp induced transfer may be contrasted e.g. with laser induced transfer, wherein the chips may tend to flip in mid-air because it is more difficult to generate a homogeneous light intensity over a larger area. For example, when the light intensity is higher on one side of the chip, delamination can occur earlier at that particular spot. Compared e.g. to robotic arms used in pick and place equipment, the throughput can be higher using light induced transfer of chips combined with soldering.

When using relatively long millisecond pulses that are typical for a high energy flash lamp, the chips may also be continuously heated, after release, while in mid air between the carrier and destination substrate. The chip can be heated relatively fast while in mid air if it is not contacting a heat sink such as the substrate and a relatively high temperature can be achieved. By modulating an intensity of the pulse, it can be adapted to different phases of the transfer, e.g. with less intensity during the transfer in mid air to prevent overheating. Light modulation can e.g. be effected by a masking device and/or control of the flash lamp. Alternatively, or in addition, by projecting the light pulse onto the chips while they are positioned on the substrate with the solder material there between, the chips can be heated and cause at least partial melting of the solder material for attaching the chips to the substrate.

The present methods can also be used for simultaneous transfer of multiple chips at once, in particular using a single light pulse. However, when different of chips are transferred, the chips can have different heating properties, e.g. caused by different dimensions (surface area and/or thickness), heat capacity, absorptivity, conductivity, number and/or size of solder bonds, etcetera. The different heating properties can make it difficult to uniformly control the transfer and soldering. To alleviate this problem, a masking device can be disposed between the flash lamp and the chips causing different light intensities in different areas of the light pulse passing the masking device. The different chips can thus be heated with the different light intensities from a (single) light pulse.

Using different light intensities, e.g. power or energy per unit area, may at least partially compensate the different heating properties of the chips to reduce a spread in temperature between the chips as a result of the heating by the light pulse. The different chips may e.g. attain a predetermined temperature in a relatively small temperature range for melting the solder material in contact therewith in a controlled manner. It will be appreciated that the present technique has the advantages of a flash lamp exposure, e.g. being relatively fast being able to expose large areas with multiple components, compatible with flexible foils and roll-to-roll processing due to the pulse duration and intensity. Furthermore, by use of the masking device, the technique can be reliable e.g. due to improved control over the heating of different chips. The masking device can also be used to prevent exposure of the substrate, e.g. at places between the chips. This may prevent damage to the substrate.

By simultaneously transmitting a single pulse to the chips via a mask pattern of the masking device, multiple chips can be exposed at different intensities. For example the mask pattern comprises filter regions that selectively attenuate different areas of the pulse impinging the mask. Accordingly, different intensities in a range up to the original intensity of the light pulse can be achieved. For example a first intensity can be set between ten and ninety percent lower or higher than a second intensity. The masking device may have a variable transmission or reflection across its surface to attenuate or otherwise selectively pass part of the light to the chips. For example, the masking pattern may have a variable transmission, reflection, and/or absorption coefficient.

The masking device can be based on reflection and/or transmission and may comprise e.g. a fixed or variable mask pattern. For example, a variable mask pattern can be achieved by electronic control of digital mirrors, LCD, or other tuneable optics. A variable mask pattern may be formed e.g. by a grid of pixels that can switch their transmission coefficient depending on a control signal. A variable light intensity can e.g. be achieved by setting multiple pixels to the same specific transmission coefficient or by using a combination of pixels with different transmission coefficients combined.

Accordingly different light intensities suitable for soldering of different chips can be attained. For example a total energy delivered to the chip per pulse can be tuned to a heat capacity of the chip, e.g. determined by its dimensions and/or material composition. For example when a chip is relatively thin, it may be heated faster than a relatively thick chip by the same energy per surface area or light intensity. A chip that has a larger surface area may receive more light from the pulse, but it may also cool down faster via a larger contact area. To calculate the desired light intensity, a heat capacity of the chip can also be normalized with respect to its surface area receiving the light.

The masking device and chip carrier can be separate or integrated, e.g. in a single foil with different transmission properties at different areas where chips are attached. In addition, by using a transparent chip carrier, the chip can be heated by the light pulse transmitted through the chip carrier. The masking device can be placed between the flash lamp and the chip, e.g. for at least partially blocking part of the light pulse from directly irradiating the substrate around the chip or for attenuating the pulse depending on the heating properties of the one or more chips. The masking device may also be integrated as part of the chip carrier, e.g. as a pattern on a flexible foil holding the chips.

Aspects of the present disclosure may also be embodied as an apparatus for soldering a chip to a substrate. The apparatus comprises a substrate handler configured to determine a location of the substrate. A carrier handler is configured to determine a location of a chip carrier with the chip attached on a side of the chip carrier facing the substrate. An alignment device and controller are configured to align the chip attached to the chip carrier with respect to a destination position of the chip on the substrate. A flash lamp is configured to generate a light pulse for heating the chip. The apparatus can e.g. be used for performing the methods described herein and vice versa. Accordingly, the heating of the chip may cause the chip to be released from the chip carrier towards the substrate wherein a solder material between the chip and the substrate is at least partially melted by contact with the heated chip for attaching the chip to the substrate.

Further aspects may be embodied as an apparatus comprising a masking device disposed between the flash lamp and the chips and configured to cause different light intensities in different areas of the light pulse passing the masking device for heating the chips, having different heating properties, with different light intensities. For example a controller can be used to control one or more parts of the apparatus in accordance with the methods described herein. Accordingly, the apparatus can be controlled such that the different light intensities may at least partially compensate the different heating properties of the chips to reduce a spread in temperature between the chips as a result of the heating by the light pulse.

The masking device may e.g. comprise a mask pattern with different filter regions. For example, two, three or more filter regions with different optical properties can be provided for selectively heating two or more chips with different light intensities while at least partially blocking light otherwise radiating the substrate between the chips. For example, different filter regions may comprise different transmission, reflection and/or absorption coefficients. Light may reach the chips e.g. via transmission through the mask or by reflection from the mask. The mask pattern area can e.g. be homogenously illuminated by optional illumination optics between the flash lamp and the mask. The apparatus may also comprise optional projection optics to image the mask pattern onto the chips. Alternatively, the mask can be placed close to the substrate and/or a relatively collimated beam of light is used to project a pattern of the mask without further optics. The projected pattern may comprise e.g. three or more different light intensities, i.e. at least two different intensities for the two different chips and a third intensity for the surrounding substrate.

A chip location device can be configured to determine locations of the chips e.g. relative to the substrate and/or masking device. For example, the chip location device may determine the location by placing the chips in a predetermined or otherwise known location. Alternatively or in addition, a chip sensor, e.g. camera, can be used to detect and determine a location of the chips. Also a size of the chips may determined at the same time as placement, or by sensor detection, e.g. using a camera. Accordingly, the position and intensity of the light determined by the mask pattern can be controlled depending on the position and size of the chips. A controller may synchronize a location of the chips with intensities of the projected mask pattern. For example, the controller may control the transmission coefficients of the mask filter regions in dependence of the respective sizes of the chips. For example, by setting a relatively lower light intensity for a part of the light pulse intended for a relatively smaller size chip, the chip can be heated to the same temperature as a relatively larger chip irradiated by a relatively higher light intensity.

For example, chips can be positioned on the substrate by a chip supply unit such as a pick and place device with the solder material there between before being illuminated by a light pulse. Alternatively, chips can be placed by illumination a chip carrier foil that releases the chips over the substrate while simultaneously heating the chip for soldering. By additionally using the masking device, chips with different heating properties, e.g. different sizes, can be contactlessly transferred from the carrier to the destination substrate. The masking device and the chip carrier can be separate devices or integrated as a single piece, e.g. comprised in a flexible foil. The chip carrier and/or masking device can be moves in synchronicity with the substrate to place the chips at the intended locations while keeping the substrate moving e.g. in a roll-to-roll process. Alternatively, or in addition, the substrate handler may slow down or stop movement of the substrate while the light is applied to the chips.

The apparatus may comprise a solder supply unit to apply the solder material to the substrate and/or chips before the chips are placed on the substrate with the solder material there between. For example, a blade coating device and/or a stencilling device can be used to apply the solder material, e.g. solder bumps to conductive tracks on the substrate where the chip is to be placed. The apparatus may comprise a a track application unit, e.g. printing device to apply conductive tracks to the substrate before the solder material is applied. Alternatively, or in addition the substrate may also be supplied with tracks already formed.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the apparatus, systems and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawing wherein:

FIGS. 1A and 1B schematically show steps for soldering a chip onto a substrate;

FIGS. 2A and 2B schematically show a further embodiment comprising mask;

FIGS. 3A and 3B schematically show a embodiments wherein a chip carrier and mask are integrated;

FIGS. 4A and 4B show steps for soldering multiple different chips at once;

FIGS. 5A and 5B schematically show heating a chip with a tuneable mask;

FIGS. 6A and 6B schematically show embodiments for stages of soldering chips onto a substrate using a roll-to-roll process.

DESCRIPTION OF EMBODIMENTS

In some instances, detailed descriptions of well-known devices and methods may be omitted so as not to obscure the description of the present systems and methods. Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that the terms “comprises” and/or “comprising” specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise.

The description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and/or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

FIGS. 1A and 1B schematically illustrate an embodiment for transfer and soldering of a chip 1 a to a substrate 3.

According to one aspect, the figure illustrates a method for soldering a chip 1 a to a substrate 3. A chip carrier 8 is provided between a flash lamp 5 and a substrate 3. A chip 1 a is attached to the chip carrier 8 on a side of the chip carrier 8 facing the substrate 3. A solder material 2 is disposed between the chip 1 a and the substrate 3. The flash lamp 5 generates a light pulse 6 for heating the chip 1 a. The heating of the chip 1 a causes the chip 1 a to be released from the chip carrier 8 towards the substrate 3. The solder material 2 is at least partially melted by contact with the heated chip 1 a for attaching the chip 1 a to the substrate 3.

According to another or further aspect, the figure also illustrates parts of an apparatus for soldering a chip 1 a to a substrate 3. For example, the apparatus comprises a substrate handler 4 configured to determine a location of the substrate 3. In the shown embodiment, the substrate handler 4 comprises rollers to handle e.g. a flexible substrate in a roll-to-roll process. Also types of substrate handlers are possible, e.g. a platform to hold a separate sheet or board of a substrate. Furthermore, the apparatus may comprise a carrier handler 18 configured to determine a location of a chip carrier 8 with the chip 1 a attached on a side of the chip carrier 8 facing the substrate 3. In the shown embodiment, the carrier handler 18 comprises rollers, e.g. to handle a carrier substrate 8 in a roll-to-roll process. The apparatus preferably comprises an alignment device and controller (not shown). These may be configured to align the chip 1 a attached to the chip carrier 8 with respect to a destination position 3 t of the chip 1 a on the substrate 3, e.g. electrical conducting tracks on the substrate surface. For example, the carrier substrate 8 and destination substrate 3 are aligned to moved synchronously wherein the chip 1 a is held above the tracks 3 t.

In one embodiment, the chip carrier 8 comprises a carrier substrate that is transparent to the light pulse 6, wherein the chip 1 a is heated by the light pulse 6 transmitted through the chip carrier 8. In a further embodiment, the chip carrier 8 comprises a transparent polymer film or a transparent glass substrate with a sacrificial adhesion layer. For example, the chip carrier 8 comprises so-called “standardized (blue) transparent polymer film” on which the silicon wafer is typically placed. This means that in principle the manufacturer of these cut wafers does not have to change their processing. Also other chip carrier substrates can be used, e.g. so-called “purple adhesive tape”. Preferably thin (e.g. silicon) chips of less than 50 microns thick are used to facilitate heat transport from the top to the base in order to solder.

In one embodiment, the light 6 a of the light pulse 6 causes decomposition of an adhesive material 8 a between the chip carrier 8 and the chip 1 a thereby releasing the chip 1 a from the chip carrier 8. The adhesive material may be part of the chip carrier 8 or a separate adhesive layer is formed between the chip and carrier. In one embodiment, the chip 1 a is at least partially transferred by and/or along a gravitational direction towards the substrate 3. Alternatively, or in addition, the release from the chip carrier 8 may cause the chip 1 a to have an initial velocity towards the substrate 3. For example, decomposition of adhesive material 8 a causes gas formation that launches the chip 1 a towards the substrate 3. For example, the rapid gas formation may provide the chip 1 a with an initial impulse.

In one embodiment, the chip 1 a is attached to the chip carrier 8 at a distance Z of at least 50 micrometer, preferably at least 100 micrometer from the substrate 3. At closer distances, the chips may start accidentally contacting the substrate 3 before being transferred. In another or further embodiment, the chip 1 a is at a distance Z of at most one millimetre, preferably at most 500 micrometer from the substrate 3. At larger distances, control over the positioning of the chip may deteriorate. For example, a gap between the chip and the destination substrate (including any conducting tracks and/or solder bumps) is at 125 micrometer. This may provide alignment accuracy of about 10 microns. For example, the substrate 3 comprises a polyimide with twelve micron thick copper tracks and solder bumps. Also other distances are possible, depending on the desired amount of control over the positioning of the chip. The distance Z can be measured between the facing surfaces of the substrates 3 and 8 or, alternatively, between the facing surface of the (thickest) chip and the contact points on the substrate 3, including any solder material there between. In the latter case the distance Z is a measure of the distance that the chip can travel between the carrier and the destination substrate.

In one embodiment, the transmitted light 6 a of the light pulse 6 continues to irradiate the chip 1 a while it is in transit (not shown) over a distance Z between the chip carrier 8 and the substrate 3. In another or further embodiment, the transmitted light 6 a of the light pulse 6 continues to irradiate the chip 1 a while it is positioned on the substrate 3 (FIG. 1B). In another or further embodiment, an intensity Ia of the light impinging the chip 1 a is modulated as a function of time. For example, the light intensity Ia is higher at a moment when the chip is released from the chip carrier 8 than during a time of transit of the chip 1 a between the chip carrier 8 and the substrate 3, and wherein the light intensity Ia is increased after the transit when the chip contacts the solder material 2 on the substrate 3. In another or further embodiment, the light intensity of the light 6 a is higher at a moment when the chip is released from the chip carrier 8 than during a time of transit of the chip 1 a between the chip carrier 8 and the substrate 3. In another or further embodiment, the light intensity of the light 6 a is higher at a moment when the chip contacts the solder material 2 than during a time of transit of the chip 1 a between the chip carrier 8 and the substrate 3. In another or further embodiment, the light intensity of the light 6 a is higher at a moment when the chip is released from the chip carrier 8 than at a moment when the chip contacts the solder material 2 on the substrate 3. For example, light modulation is caused by the flash lamp 5 and/or a masking device between the flash lamp and substrate

Preferably, a millisecond light pulse 6 is used e.g. produced by a (pulsed) Xenon flash lamp. A typical pulse may deliver a total energy between 1 and 20 J/cm² e.g. in a pulse time between 0.5 to 10 ms. For example, a Xenon or other high intensity flash lamp can be used, e.g. with a pulse length of 2 ms and pulse intensity of 10 J/cm². A flash lamp, also called flashtube, typically comprises an electric arc lamp configured to produce intense (incoherent) light for short durations, e.g. light pulses having a pulse length between 500 microseconds and 20 milliseconds. Also shorter or longer pulses may be possible. Flashtubes are for example made of a length of glass tubing with electrodes at either end and are filled with a gas that, when triggered, ionizes and conducts a high voltage pulse to produce the light. For example a Xenon flash lamp can be used to produce high light intensities sufficient to irradiate a chip surface and at least partially melt a solder material in contact with the chip e.g. by heat conducted through the chip.

FIGS. 2A and 2B schematically illustrate a further embodiment, wherein a masking device 7 is disposed between the flash lamp 5 and the chip 1 a. The masking device 7 may at least partially block part of the light pulse 6 from directly irradiating the substrate 3 and/or chip carrier 8 around the chip. In one embodiment, the masking device 7 comprises a masking pattern 7 a,7 c disposed between the flash lamp 5 and the chip 1 a. For example, the masking pattern 7 a,7 c is configured to selectively pass light 6 a of the light pulse 6 to the chip 1 a and block other light not impinging the chip 1 a from reaching the substrate 3. For example, in one embodiment, a high intensity pulsed xenon flash lamp is used in combination with a (lithographical) mask to pattern the light pulse impinging the chips. In another or further embodiment, the chip carrier 8 and/or masking device 7 are comprised in a flexible foil, e.g. having variable transmission for different areas where chips are attached.

FIG. 3A schematically illustrates an embodiment, wherein the masking device 7 is integrated as a layer on top of the chip carrier substrate 8.

FIG. 3B shows another embodiment with even further integration of the masking device 7 and chip carrier 8 in a single substrate. In one embodiment, the masking device 7 comprises a foil with varying degrees of transparency depending on where chips are attached. For example, the area 7 a may transparent for providing the full intensity of the pulse to a first chip 7 a, and another area (not shown) may be partially opaque or semi-transparent for attenuating light of the pulse 6 e.g. impinging a second chip, e.g. having a lower heat capacity per illuminated area.

FIGS. 4A and 4B schematically show an embodiment wherein a plurality of chips 1 a, 1 b are simultaneously transferred from the chip carrier 8 to the substrate 3 and soldered to the substrate 3. Advantageously, the transfer and soldering of one or more chips can be effected by a single light pulse 6.

In one embodiment, two or more different chips 1 a, 1 b having different heating properties C1,C2 are attached to the chip carrier 8. In another or further embodiment, a masking device 7 is disposed between the flash lamp 5 and the chips 1 a, 1 b causing different light intensities Ia,Ib in different areas 6 a,6 b of the light pulse 6 passing the masking device 7. Accordingly, the chips 1 a, 1 b can be heated with different light intensities Ia,Ib for at least partially compensating the different heating properties C1,C2 to reduce a spread in temperature between the chips as a result of the heating by the light pulse 6.

In one embodiment, the apparatus comprises a substrate handler 4 configured to determine a location of the substrate 3 and/or chips 1 a, 1 b. For example, the apparatus comprises a sensor (not shown) configured to determine a location of the chips 1 a, 1 b with respect to the substrate 3.

In one embodiment, the light pulse 6 is simultaneously transmitted to the chips 1 a, 1 b via a mask pattern 7 a,7 b,7 c of the masking device 7. For example, the mask pattern 7 a,7 b,7 c comprises a first filter region 7 a passing a first part 6 a of the light pulse 6 with a first light intensity Ia to a first chip 1 a; and a second filter region 7 b passing a second part 6 b of the light pulse 6 with a second light intensity Ib to a second chip 1 b, wherein the first light intensity Ia is different than the second light intensity Ib. Light intensity is measured e.g. per unit area of the chip surface receiving the part of the light pulse.

In one embodiment, the first chip 1 a has a first heat capacity C1 and the second chip 1 b has a second heat capacity C2 different from the first heat capacity C1. For example in the shown embodiment, the first chip 1 a is thinner than the second chip 1 b. An object's heat capacity is defined e.g. as the ratio of the amount of heat energy transferred to an object and the resulting increase in temperature of the object. Heat capacity may be larger for larger objects or for objects containing a material with larger specific heat capacity (per unit mass) or volumetric heat capacity (per unit volume). Preferably, the different light intensities Ia,Ib at least partially compensate a difference in heat capacity C1,C2 or other difference in heating property between the different chips 1 a, 1 b for reducing a spread in temperature of the chips heated by the light pulse 6.

In one example, two components having different thicknesses and surface areas may need a different input energy for soldering components. For example, a lower thickness and surface may result in a low heat capacity leading to a relatively high temperature increase per input energy unit, while on the contrary a reduced number of solder bonds may require lower input energy for soldering corresponding bonds. Using a mask with corresponding transmittance filters, exposure fluence can be locally tuned allowing for different chips to be soldered with a single pulse. Filters could be for example either with fixed or configurable transmittance.

In one embodiment, the apparatus comprises optional illumination optics (not shown) configured to homogeneously illuminate an area of the masking device 7 with the mask pattern 7 a,7 b,7 c. In another or further embodiment, the apparatus comprises optional projection optics (not shown) configured to project an image of the mask pattern 7 a,7 b,7 c onto the chips 1 a, 1 b. In the embodiment shown, the transmitted light 6 a,6 b of the light pulse 6 is projected onto the chips 1 a, 1 b while they are positioned on the substrate 3 with the solder material 2 there between thereby heating the chips 1 a, 1 b. The heated chips 1 a, 1 b may cause the at least partial melting of the solder material 2 for attaching the chips 1 a, 1 b to the substrate 3 (after resolidification).

In one embodiment, the masking device 7 comprises a mask pattern 7 a,7 b,7 c configured to selectively transmit the light pulse 6 to the chips 1 a, 1 b. In another or further embodiment, the mask pattern 7 a,7 b,7 c comprises a first filter region 7 a having a first transmission coefficient Ta configured to transmit light 6 a of the light pulse 6 with a first light intensity Ia to a first chip 1 a for melting a solder material 2 between the first chip 1 a and the substrate 3; and a second filter region 7 b having a second transmission coefficient Tb configured to transmit light 6 b of the light pulse 6 with a second light intensity Ib to a second chip 1 b for melting a solder material 2 between the second chip 1 a and the substrate 3. In a further embodiment, the first transmission coefficient Ta is different than the second transmission coefficient Tb for simultaneously irradiating the chips 1 a, 1 b with different light intensities Ib,Ib. The transmission coefficient is a measure of how much of an electromagnetic wave (light) passes a surface or an optical element. For example, transmission coefficients can be calculated for either the amplitude or the intensity of the wave. Either is calculated by taking the ratio of the value after the surface or element to the value before.

In one embodiment, the filter regions 7 a,7 b,7 c of the mask pattern are controllable to tune the transmission coefficients Ta,Tb. For example, the mask pattern 7 a,7 b,7 c is formed by tunable optics, e.g. a grid of digital mirrors, LCD, and/or polarizing optics. In one embodiment, the mask pattern 7 a,7 b,7 c comprises a third filter region 7 c having a third transmission coefficient Tc configured to substantially block part of the light pulse 6, e.g. a part that would otherwise be directly projected onto the substrate 3.

In one embodiment, the masking device 7 comprises photolithographed metal on glass. For example, aluminium or chrome is used to vary the light intensity of the pulse in two, three, or more different intensities. In one embodiment, the masking device 7 comprises a cooling device (not shown), e.g. water cooling to handle (partial) absorption of high energy light pulses.

FIG. 5A shows an apparatus for soldering a chip 1 a onto a substrate 3, wherein the masking device 7 comprises a first filter region 7 a with pixels 7 p having a first transmission coefficient Ta to transmit light 6 a of the light pulse 6 with the first light intensity Ia to the first chip 1 a.

FIG. 5B shows an apparatus for soldering a chip 1 a onto a substrate 3, wherein a filter region 7 a transmitting part 6 a of the light pulse 6 to a chip 1 a comprises multiple pixels 7 p having different transmission coefficients, wherein the first light intensity Ia is determined by a combination light intensities transmitted by the pixels 7 p having different transmission coefficients. For example, a dithered pattern of pixels can be used to reduce the overall or average intensity Ia of light 6 a impinging the chip 1 a. The embodiments of FIGS. 5A and 5B can e.g. be used in combination with a chip carrier between the masking device 7 and substrate 3.

FIG. 6A illustrates an embodiment of an apparatus for transfer and soldering of chips in a roll-to-roll fabrication process. In the shown embodiment, the substrate handler 4 comprises rolls to handle the foil substrate 3 which may be flexible. In another or further embodiment, the chip carrier 8 and/or masking device 7 are also comprised in a flexible foil. In another or further embodiment, the chip carrier 8 and/or masking device 7 are configured to move in synchronicity with the substrate 3. In one embodiment, the flash lamp 5 is configured to deliver a single pulse 6 to transfer and solder multiple chips 1 a, 1 b having possibly different sizes or other heating properties.

In the embodiment the apparatus comprises an alignment device 12 (e.g. camera or other sensor) and a controller 15. In another or further embodiment, the apparatus comprises a controller 15 configured to control the alignment device 12, substrate handler 4 and/or substrate handler 4. For example, the controller 15 is programmed to align the chips attached to the chip carrier 8 with respect to destination positions on the substrate 3. Alternatively, or in addition, the controller 15 is programmed to align the different light intensities of the different areas 6 a,6 b of the light pulse 6 with locations of the different chips 1 a, 1 b.

FIG. 6B illustrates another embodiment of an apparatus for soldering of chips in a roll-to-roll fabrication process. In the embodiment, a variably tuned masking device 7 is used at a fixed position e.g. above a chip carrier 8 held by a suitable carrier handler (not shown)

In one embodiment, the apparatus comprises a controller 15 configured to variably tune the light intensities Ia,Ib in dependence of the respective sizes of the chips 1 a, 1 b. In one embodiment, a controller 15 is configured to determine locations of the chips 1 a, 1 b from a chip location device and to control the masking device 7 and/or substrate handler 4. For example, the controller 15 is programmed to align the different light intensities of the different areas 6 a,6 b of the light pulse 6 with locations of the different chips 1 a, 1 b. For example, the controller 15 is programmed to control the transmission coefficients of the filter regions of the masking device 7 in dependence of the respective sizes of the chips 1 a, 1 b.

In one embodiment, the controller 15 is programmed to set a relatively high light intensity for a part 6 b of the light pulse 6 intended for a chip 1 b having a relatively high heat capacity per illuminated area, e.g. a relatively thick chip. In another or further embodiment, the controller 15 is programmed to set a relatively low light intensity for a part 6 a of the light pulse 6 intended for a chip 1 a having a relatively low heat capacity per illuminated area, e.g. a relatively thin chip. In one embodiment, the controller 15 is programmed to set a relatively higher transmission coefficient Tb for a part 6 b of the light pulse 6 intended for a relatively larger size chip 1 b and a relatively lower transmission coefficient Ta for a part 6 a of the light pulse 6 intended for a relatively smaller size chip 1 a.

In one embodiment, the apparatus comprises a sensor 12 (e.g. camera) configured to detect a location the tracks 13 on the substrate 3. The sensor 12 may provide feedback to the controller 15 which can be used to align the position of the chips and/or intensity of the light. In one embodiment, the substrate handler 4 is configured to slow down or stop movement of the substrate 3 while the light 6 a,6 b is applied to the chips 1 a, 1 b.

In one embodiment, the apparatus comprises a solder supply unit 9 configured to apply the solder material 2 to the substrate 3 and/or chips 1 a, 1 b before the chips 1 a, 1 b are placed on the substrate 3 with the solder material 2 there between. For example, the solder supply unit 9 comprises a blade coating device and/or a stencilling device.

In one embodiment, the apparatus comprises a track application unit 10 configured to apply, e.g. print, conductive tracks to the substrate 3 before the solder material 2 is applied, wherein, in use, the chips 1 a, 1 b are electrically connected to the tracks.

For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. For example, it will be clear that the devices described with reference to FIGS. 6A and 6B can also be used in other embodiments than roll-to-roll processing. For example, the controller 15 can also be used to control placement of chips on a fixed substrate Also the other devices 10, 9, 12 as described herein can be applied in other embodiments, alone or in any combination, possibly under separate or shared control of a controller 15 as described herein. The controller may be programmed with software that allow it to execute operational steps in accordance with methods as described herein with reference to any of the embodiments.

Also other combinations will be readily apparent to the skilled artisan having the benefit of the present disclosure for achieving a similar function and result. For example electronic and mechanical components may be combined or split up into one or more alternative components. The various elements of the embodiments as discussed and shown offer certain advantages, such as fast and reliable soldering of chips and/or contactless transfer of chip. Of course, it is to be appreciated that any one of the above embodiments or processes may be combined with one or more other embodiments or processes to provide even further improvements in finding and matching designs and advantages. It is appreciated that this disclosure offers particular advantages to roll-to-roll processing, and in general can be applied for any application wherein chips are soldered.

Finally, the above-discussion is intended to be merely illustrative of the present systems and/or methods and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. The specification and drawings are accordingly to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. In interpreting the appended claims, it should be understood that the word “comprising” does not exclude the presence of other elements or acts than those listed in a given claim; the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements; any reference signs in the claims do not limit their scope; several “means” may be represented by the same or different item(s) or implemented structure or function; any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. In particular, all working combinations of the claims are considered inherently disclosed. 

1. A method for soldering a chip to a substrate, the method comprising: providing a chip carrier between a flash lamp and the substrate, wherein the chip is attached to the chip carrier on a side of the chip carrier facing the substrate, and wherein a solder material is disposed between the chip and the substrate; and generating a light pulse with the flash lamp for heating the chip, wherein the light pulse heating of the chip causes the chip to be released from the chip carrier to transfer contactlessly towards the substrate, and wherein the solder material is at least partially melted by contact with the chip, after the chip is heated by the light pulse, for attaching the chip to the substrate.
 2. The method according to claim 1, wherein a transmitted light of the light pulse from the flash lamp continues to irradiate the chip while the chip is contactlessly in transit over a distance between the chip carrier and the substrate.
 3. The method according to claim 2, wherein an intensity of the transmitted light impinging the chip is modulated as a function of time by at least one of the group consisting of: a controlling of the light pulse of the flash lamp, and a masking device between the flash lamp and chip carrier, wherein, by said modulation, an intensity of the transmitted light is higher at a moment when the chip is released from the chip carrier than during a time of transit of the chip between the chip carrier and the substrate.
 4. The method according to claim 3, wherein the transmitted light of the light pulse continues to irradiate the chip while it is positioned on the substrate, and wherein an intensity of the transmitted light is increased after the transit when the chip contacts the solder material on the substrate.
 5. The method according to claim 1, wherein the chip carrier comprises a carrier substrate that is transparent to the light pulse, and wherein the chip is heated by the light pulse that is transmitted through carrier substrate of the chip carrier.
 6. The method according to claim 1, wherein the light of the light pulse causes decomposition of an adhesive material between the chip carrier and the chip thereby releasing the chip from the chip carrier.
 7. The method according to claim 1, wherein the chip is attached to the chip carrier at a distance from the substrate, and wherein the distance is between 50 and 500 micrometer.
 8. The method according to claim 1, wherein a masking device comprises a masking pattern, wherein the masking device is disposed between the flash lamp and the chip, and wherein the masking pattern is configured to selectively pass light of the light pulse to the chip.
 9. The method according to claim 1, wherein a plurality of chips are simultaneously transferred from the chip carrier to the substrate and soldered to the substrate.
 10. The method according to claim 1, wherein the transfer and soldering of one or more chips is effected by a single light pulse.
 11. The method according to claim 1, wherein two or more different chips having different heating properties are attached to the chip carrier; and a masking device is disposed between the flash lamp and the two or more different chips to cause different light intensities in different areas of the light pulse passing the masking device, and thereby heating individual ones of the two or more different chips with different light intensities, and wherein the different light intensities at least partially compensate the different heating properties to reduce a spread in temperature of the two or more different chips as a result of heating by the light pulses from the flash lamp.
 12. The method according to claim 1, wherein the substrate comprises a flexible foil in a roll-to-roll process.
 13. An apparatus for soldering a chip to a substrate, the apparatus comprising: a substrate handler configured to determine a location of the substrate; a carrier handler configured to determine a location of a chip carrier with the chip attached on a side of the chip carrier facing the substrate; an alignment device configured to align the chip attached to the chip carrier with respect to a destination position of the chip on the substrate; a flash lamp configured to generate a light pulse for heating the chip, wherein the light pulse heating of the chip causes the chip to be released from the chip carrier towards the substrate, and wherein a solder material between the chip and the substrate is at least partially melted by contact with the heated chip for attaching the chip to the substrate.
 14. The apparatus according to claim 13, further comprising a masking device disposed in use between the flash lamp and the chip carrier, wherein the masking device is configured to cause different light intensities in different areas of the light pulse passing the masking device, and thereby heating individual ones of multiple different chips on the chip carrier, having different heating properties, with different light intensities.
 15. The apparatus according to claim 13, further comprising a solder supply unit configured to apply the solder material to the substrate and/or chip before the chip is transferred to the substrate with the solder material there between.
 16. The apparatus according to claim 13, comprising: a controller configured to modulate an intensity of light impinging the chip as a function of time by control of the flash lamp and/or a masking device positioned between the flash lamp and chip carrier; wherein the controller is configured to cause the light pulse to continue to irradiate the chip while the chip is contactlessly in transit over a distance between the chip carrier and the substrate; wherein, by modulating the intensity of light impinging the chip, the intensity of light impinging the chip is higher at a moment when the chip is released from the chip carrier than during a time of transit of the chip between the chip carrier and the substrate. 